The present invention relates to offshore oil extraction and, more particularly, to an oil extracting installation employing flexible pipes for transporting fluids under pressure, such as hydrocarbons, coming from underwater wellheads.
The pipes employed for transporting hydrocarbons may become soiled and clogged due to the formation of a deposit on the inner surface of the pipes, which happens frequently when the hydrocarbons have a high paraffin content.
In this case, cleaning of the pipes, also called "pigging", may be carried out periodically by exerting mechanical action on the inner surface of the pipes.
In order to conduct this cleaning operation, the underwater wellheads are isolated from the flexible pipe sections serving for transporting the hydrocarbons up to the surface, and these flexible pipe sections are connected to one another so as to form a loop which makes it possible, from a surface extracting installation, to inject a cleaning device at one end of the loop and to recover this cleaning device at the other end of the loop.
In order to make it possible to isolate the underwater wellheads more easily from the flexible pipe sections serving for transporting the hydrocarbons up to the surface, it is known for the connecting manifolds, allowing a sealed connection to be made between the pipes of short length and small diameter coming from the wellheads and the flexible pipe sections of great length and larger diameter serving for transport up to the surface, to be arranged on mounting plates resting on the ocean bed.
Using such mounting plates makes it easier to connect the pipes of different diameters, for example make the connection between a flexible pipe having an inside diameter of 6" and a pipe having an inside diameter of 4", inasmuch as it would be difficult for a manifold to be incorporated for this purpose on the wellhead itself in view of its structure.
Moreover, these mounting plates may support the valves which make it possible to isolate the wellheads and put the various flexible pipe sections in communication with one another in order to form the loop necessary for the cleaning operation.
In order, during the cleaning operation, to connect two flexible pipe sections employed during extraction for conveying the hydrocarbons from the respective wellheads up to the surface extracting installation, it has been proposed to use a mounting plate supporting a rigid U-shaped union connected at its ends to two flexible pipe sections.
The U-union is isolated by means of a valve during extraction from the well, so that the hydrocarbons produced by each of the wellheads pass through two separate flexible pipe sections up to the surface extracting installation; for cleaning to be carried out, each wellhead is isolated and the two flexible pipe sections are put in communication by means of the U-union, thus making it possible to form a loop allowing a cleaning device to pass through the two flexible pipe sections.
However, the use of a mounting plate supporting a U-union connected to two juxtaposed flexible pipe sections makes the installation difficult and costly to install, inasmuch as one of the two flexible pipe sections is connected to a second mounting plate which has to be put under water while the unwinding of the two flexible pipe sections by chain is being carried out at the same time.
A second solution would involve placing the mounting plates on the bed and connecting the flexible pipes to the manifolds in situ.
However, such a solution is not easy to put into practice, inasmuch as it is difficult for elements to be assembled to be displaced horizontally on the ocean bed.